The present invention relates to analytical chemistry and, more particularly, to conductivity detectors used, for example, to detect sample fluid components as they flow in a channel through a detection region. A major objective of the invention is to provide for comparative contactless conductivity detection.
Much of modern progress in the medical, pharmaceutical, environmental, industrial-process, forensic, and other sciences can be attributed to advances in analytical chemistry. One important class of analytical tools separates components of a sample fluid (typically, a mixture of sample components and non-sample components such as carriers, buffers, and surfactants) by moving them at different rates along a separation channel. Once the components are separated, it is usually desirable to quantify, and, perhaps, identify the components. This typically requires detection of the components. Detectors are available to monitor certain parameters, such as conductivity, fluorescence, or absorption of ultraviolet (UV) electromagnetic energy as the components pass through a detection region.
Conductivity detection is appealing for electrophoresis, in which components are separated by an electric field according to their electrophoretic mobilities. Components separated by electrophoresis necessarily have a measurable electrical conductivity associated with their electrophoretic mobilities. More generally, conductivity detection is useful for detecting the components with measurable conductivity regardless of how they arrive at the detector region.
Conductivity detection can be implemented by locating electrodes on the interior walls of an electrophoretic channel, in direct contact with the sample fluid. Typically, drive and detection electrodes oppose each other across a transverse width or diameter of the electrophoretic channel. However, since the electrodes are in contact with the sample fluid, electrochemical reactions at the electrodes can affect both the electrodes and the sample. Such interaction can cause undesirable artifacts within a run and can undermine repeatability between runs. This undesirable interaction between sample and electrodes is avoided by xe2x80x9ccontactlessxe2x80x9d conductivity detection.
In contactless conductivity detection, electrodes are capacitively coupled to the sample fluid through a channel wall. To this end, the electrodes can be formed on the exterior surface of the channel wall. Since the electrodes are not in contact with sample fluid, artifacts due to chemical interactions at the electrodes are eliminated and reproducibility is improved.
Contactless conductivity detection is taught by Jose A. Fracassi da Silva and Claudimir L. do Lago xe2x80x9cAn Oscillometric Detector for Capillary Electrophoresisxe2x80x9d, Analytical Chemistry, vol. 70, 1998, pp. 4339-4343; Jirxc3xad Vacik, Jirxc3xad Zuska and Iva Muselasova, xe2x80x9cImprovement of the Performance of a High-Frequency Conductivity Detector for Isotachophoresisxe2x80x9d Journal of Chromatography, 17,322, 1985, 5 pages; Andress J. Zemann, Erhard Schnell, Dietmar Volger, and Gxc3xcnther K. Bonn, xe2x80x9cContactless Conductivity Detection for Capillary Electrophoresisxe2x80x9d Analytical Chemistry, V.70, 1998, pp. 563-567. In addition, an anti-synchronously driven contactless conductivity detector is the subject of commonly owned U.S. patent application Ser. No. 09/576,690 filed May 23, 2000, entitled xe2x80x9cSample-analysis system with anti-synchronously driven contactless conductivity detectionxe2x80x9d by Gary B. Gordon and Tom A. vande Goor.
All of the foregoing contactless conductivity detectors are designed to characterize the conductivity profile over time of a fluid as it flows through a detection region of a fluid channel. However, it is often desirable to compare the conductivity profiles of two fluids. Generally, the profile of a sample fluid can be compared with the profile of a reference fluid to remove xe2x80x9cuninterestingxe2x80x9d profile features due to the carrier, buffer, surfactant, etc. In industrial-process applications, it is often important to determine whether or not the composition of a process fluid has changed. In forensic applications, it is often desirable to determine whether or not two samples have the same composition.
Comparative conductivity profiles can be obtained by subtracting two independently obtained xe2x80x9cabsolutexe2x80x9d fluid conductivity profiles. However, high-level background signals corresponding to non-sample components can make it difficult to obtain precise conductivity profiles. Furthermore, errors in the absolute profiles are exacerbated when they are subtracted to obtain the desired comparison. What is needed is a system that can provide more error-free comparative conductivity profiles.
The present invention provides a comparative contactless-conductivity detection system with at least two channel assemblies, each with a fluid channel, drive electrodes, and detection electrodes. The drive electrodes are driven so that detection signals are induced at the detection electrodes; these detection signals are combined to provide a detector signal corresponding to a difference in conductivities between the two fluids. Preferably, drive electrodes are driven anti-synchronously and the detection electrodes are electrically connected so that the detection signals sum at a common node. In view of the anti-synchronous drive, the sum corresponds to the difference between the conductivities of the fluids. In other words, the detector signal represents the comparative conductivity of one the fluids relative to the other.
In a typical application, a first channel carries a sample fluid to be characterized, while a second channel carries a reference fluid with a known conductivity. The reference fluid conductivity can be constant or can vary in a known manner over time. For example, the reference fluid can be the same chemical as the buffer used in the sample fluid. The comparative signal would represent the sample components without the background associated with the buffer. In an isotachophoresis application, the reference fluid can have its conductivity vary spatially in a manner that generally mimics the sample fluid to reduce the background against which the signal of interest is to be read.
Alternatively, the reference fluid can be another sample of known or unknown composition. The comparative conductivity detector provides a clear indication when two samples have the same composition. In addition, the comparative conductivity detector would help pinpoint small differences in composition. Known reference fluids can be used to confirm a match between a sample and reference chemical. Reference fluids of unknown composition can be used in industrial-process and forensic applications where the similarity or identity of samples is the issue rather than the composition of the samples.
The invention applies to sample fluids whether or not the sample component fluids have been separated by some separation technique. For applications in which the sample-fluid components have been separated, the reference-fluid components may or may not be subject to the same separation forces as the sample-fluid components. If the reference fluid is intended to vary over time in a controlled fashion, the reference fluid is in general not subjected to separation forces. On the other hand, the reference fluid should be subjected to the same separation forces as the sample fluid if component-by-component comparison is to be achieved.
The present invention provides for fabricating the sample and reference channels in a monolithic substrate using, for example, integrated-circuit manufacturing techniques. The resulting micro-fluidic platform provides a remarkably convenient way to create a reference channel that is precisely matched to the sample channel. In addition, the channels can be in close proximity, thereby minimizing artifacts due to differences in ambient conditions at the channels. For example, the temperatures of the sample and reference fluids can be made substantially identical, thereby removing temperature as a noise source in the measurement. Including the reference channel within the micro-fluidic platform also enhances the ease with which one can change and control the reference fluid.
In summary, comparative conductivity detection provides a stronger signal of interest by reducing the influence of high background signal levels. When implemented using precision monolithic manufacturing techniques, the invention can minimize artifacts due to environmental changes. Furthermore, the invention provides considerable flexibility in selecting reference fluids to match the intended application. These and other features and advantages are apparent from the description below with reference to the following drawings.